The present invention relates to the preparation of ruthenium compounds useful as precursors of ruthenium catalysts. It concerns more particularly a process for the preparation of a ruthenium compound of formula
{Ru(dienyl)2}xe2x80x83xe2x80x83(I)
wherein xe2x80x9cdienylxe2x80x9d represents a substituted pentadienyl or cycloheptadienyl group.
A compound of the invention, bis(2,4-dimethyl-2,4-pentadienyl)-ruthenium or {Ru(DMPD)2}, is a known product useful for the preparation of a great number of ruthenium catalysts, in particular compounds useful in asymmetric catalysis. Although a prior synthesis of this compound has been described by L. Stahl et al. In Organometallics 1983, 2, 1229-1234, practical working of this known process showed that it is not useful for industrial exploitation as it provides the desired compound in low yields, below 40%
In order to overcome the limitation of the prior art process, the invention relates more particularly to a new process for the preparation of a ruthenium compound of formula
{Ru(dienyl)2}xe2x80x83xe2x80x83(I)
wherein xe2x80x9cdienylxe2x80x9d represents a substituted pentadienyl or cycloheptadienyl group, which process comprises reacting dichloro(2,7-dimethylocta-2,6-dien-1,8-diyl)-ruthenium with an appropriate diene in the presence of:
1) a primary or secondary alcohol capable of reducing Ru(IV) to Ru(II); and
2) a carbonate of an alkaline metal.
By xe2x80x9can appropriate dienexe2x80x9d it is meant here the diene which, for a particular compound (I), provides the corresponding dienyl group.
According to a preferred embodiment of the invention the reaction is carried out in the presence of a catalytic amount of acetonitrile.
The process of the invention is particularly useful for preparing compounds (I) wherein xe2x80x9cdienylxe2x80x9d stands for a 2,4-dimethyl-2,4-pentadienyl, 2,3,4-trimethyl-2,4-pentadienyl, 2,4-dimethyl-1-oxa-2,4-pentadienyl or 2,4-cycloheptadienyl group.
Dichloro(2,7-dimethylocta-2,6-dien-1,8-diyl)-ruthenium, used as the starting compound in the process of the invention, is a known compound. Several synthesis of this compound have in fact been described, e.g. by L. Porri et al., Tetrahedron Lett. 1965, 4187-4189, D. N. Cox et al., Inorg. Chem. 1990, 29, 1360-1365 and J. G. Toerien et al., J. Chem Soc. Dalton Trans. 1991, 1563-1568. However, to our knowledge, there has never been any report or suggestion of the use of this known compound for the preparation of compounds of formula (I).
Primary or secondary alcohols suitable for use in the process of the invention include ethanol, methanol and isopropanol, amongst other. Preferably, ethanol will be used.
Carbonates of alkaline metals appropriate for the process of the invention include lithium, sodium, and potassium carbonates in particular. Preferably lithium carbonate will be used.
According to an advantageous embodiment of the invention, dichloro(2,7-dimethylocta-2,6-dien-1,8-diyl)-ruthenium is reacted with 2,4-dimethyl-1,3-pentadiene, in the presence of ethanol, acetonitrile and lithium carbonate to provide bis(2,4-dimethyl-2,4-pentadienyl)-ruthenium or {Ru(DMPD)2}.
Unlike the prior art process, the process of the instant invention makes it possible to prepare compounds (I) in high yields, in many cases above 90%. It can be carried out in simple equipment and it does not require particular temperature or pressure conditions.